Purification of toluic acids



United States Patent PURIFICATION 9F TOLUIC ACIDS Chester M. Himel,Menlo Park, Calii, assignor to Richfield Oil Corporation, Los Angeles,Cali'r., a corporation of Delaware No Drawing. Application July 1, 1952,Serial No. 296,716

3 Claims. (Cl. 260-525) My invention relates to the production of toluicacids in a purified form. More specifically, my invention is concernedwith the separation of close-boiling neutral materials from toluic acidsproduced by the air oxidation of xylenes.

Toluic acids can be prepared by the oxidation of xylenes with air orother oxygen-rich gas. One commercial source of xylenes is to be foundin cracked or reformed petroleum oils. Thus, petroleum naphtha which isrich in dimethyl cyclohexanes is dehydrogenated in the presence ofhydrogen in accordance with the hydroforming or platforming processes toyield xylenes and other compounds, depending upon the purity of thenaphtha fraction processed with respect to content of dimethylcyclohexanes. The toluic acids produced are associated with neutralimpurities which have boiling points close to those of toluic acids andwhich are therefore difficult to separate from the toluic acids.

My application Serial No. 126,240, now abandoned, and that of LawrenceM. Richards, Serial No. 126,237, now abandoned, both filed November 8,1949, relate to the production of phthalic acids by the mild oxidationof xylenes to produce toluic acids and a further, more vigorousoxidation of toluic acids to produce phthalic acids, utilizing petroleumxylene fractions as starting materials. In these applications, apetroleum xylene fraction is first subjected to direct, liquid phaseoxidation with air in the presence of a catalyst which is soluble in thereaction mixture, such as a cobalt, manganese or cerium soap,particularly the toluate soap. Reaction temperatures of from about 130C. to about 190 C. are useful. The air oxidation is carried to partialcompletion, that is, from about 5 percent to about 30 percent by weightof oxygen is absorbed, based on the aromatic content of the petroleumxylene fraction. Such mild partial oxidation, while yielding a lowerconversion of the petroleum xylene fraction, favors the production oftoluic acids from the xylenes and acetophenone from any ethyl benzenepresent.

Acetophenone and partial oxidation products such as tolualdehydes areconveniently separated by fractional distillation. When such adistillation is made, there is produced a small fraction of neutraloxidation products which distills between the distillation range oftolualdehydes and the distillation range of the toluic acids. Thefraction containing the toluic acids is the last fraction taken 0Eoverhead. This fraction, however, also contains varying amounts ofneutral materials, depending upon the reaction conditions used in theair oxidation and the handling conditions during the preliminarydistillation stages. These neutral materials or neutral oils present inthe toluic acid fraction are possibly condensation products ofacetophenone and tolualdehydes as well as other oxidation by-productsand, as has been stated, have a boiling range close to, or within, theboiling range of the toluic acid isomers. The toluic acid fractionproduced from the distillation has a light yellowgreen color and acharacteristic odor. Analyses indicate ice that it contains from aboutto about 98 percent of toluic acids and traces of benzoic acid, with theremainder being the neutral oils mentioned above.

in accordance with my invention, 1 have found that the materials whichcontaminate the distilled toluic acids are also responsible for thediscoloration and the odor of the acids. Moreover, I have found thatsuch contaminants are quite soluble in saturated hydrocarbons havingfrom 3 to 8 carbon atoms per molecule, and that toluic acids are solublein such hydrocarbons to only a limited extent. I therefore utilize suchhydrocarbons to purify the toluic acids produced by the air oxidation ofthe petroleum xylene fractions by simply washing the crude distillatecomposed of the crystalline toluic acids and associated materials withone or more hydrocarbons falling within the class stated above.

This can be done in various ways. For example, the neutral impuritiescan be solvent extracted by intimately contacting the crude, crystallinetoluic acid fraction with propane, butane, pentane, isopentane, hexanes,heptanes, cyclohexane, methyl cyclopentane, dimethyl cyclopentanes,methyl cyclohexane, dimethyl cyclohexanes, or a mixture of thesecompounds. On the other hand, a process of combined solvent extractionand crystallization can be carried out by charging the molten toluicacid fraction directly from the distillation apparatus or from a toluicacids melt tank into a well-agitated excess of one or more of thesolvents just mentioned. After filtration of the crystalline, purifiedtoluic acids, the solvent containing dissolved impurities can berecovered for further use by distillation, in accordance with well knownprocedures, leaving the impurities as a kettle residue in the solventrecovery still. Preferably I use hydrocarbons having from 4 to 6 carbonatoms because of the better recoveries of toluic acids and the ease ofrecovery of the solvent. The hydrocarbons which I use are preferablyessentially saturated, but they may be in admixture with small amountsof monoolefins of the same range of carbon content.

The following examples illustrated in detail the practice of myinvention, and are to be considered not limitative thereof.

Example I A commercial xylene fraction having the following analysis:

was used (1500 grams) to charge a stirred autoclave, along with 45 gramsof cobaltous toluate catalyst. The reactor was stirred and pressured to500 p. s. i. g. with air and heated to 150 C. When reaction temperaturewas reached, 40 cu. ft. of air (measured at room temperature andpressure) was passed into the reactor over a 39-minute period. Efiluentgases were released through a back pressure regulator which maintainedthe pressure in the autoclave at 500 p. s. i. g. The efliuent, whichweighed 1700 grams, was processed to remove the catalyst and water.

The xylene solution was then divided into a 300-g-ram aliquot which wasdistilled to give 196 grams of xylene, 36 grams B. P. 70-130 C./5 mm.and 65 grams of toluic acid fraction B. P. 130-5 C./5 mm. The toluicacid fraction was a light yellow color and had a neutral equivalent of146, indicating the presence of 7 percent neutral impurities. The yellowsolid was ground and extracted once with cc. of petroleum ether (B.35-60 C.) to give a pure white residue which had a neutral equivalent of140. A second extraction gave 55 grams of white crystals having aneutral equivalent of 136, corresponding to pure toluic acids.

Example 11 4 The pentane treatment not only efiectivelyremovedcontaminating materials from this product but also allowedseparation into two fractions having different para-toluic acid content.This is of value in the preparation of a feed stock for the productionof pure para-toluic acid.

While the foregoing is illustrative of the practice of this invention,the. actual washing. or extraction step is susceptible to obviousvariations. Thus, the relative amounts of crude toluic acids: andsolvent medium-used is not critical, but rather can be varied widely; Inactualpractice, the relative amounts of crude ltoluic acids and solventmedium used will depend uponthe extent to which the toluic.acidsarecontaminated, upon the particular solvent used, and upon otherfactors. Fun-ther- Start 15!; Extraction 2nd Extraction Sample gms. N. Ecolor gms. N. color gms; N. E. color 100 Mi. yellow s0. 137 white. 76.136 white. 100 144' brown. 85 140' tan 75' 138 beige. 100 140ll,.bl'.OW11' 83' 137' vvhite 74 136 white. 100 136.5 In. yellow as 7136 white After a third extraction of sample B, there remained M! 2,5more, although the extraction was carried out at room;

grams of acids which had a neutral equivalent of 136 andlwhichwere beigein'color. A150,, a- SSS-gram sample of A was then extracted with 358ml.. of pentane to give 295 grams of white crystals with a neutralequivalent of 136.5. 7

Example 111 A toluic acid product from a low xylene content charge stockwas subjected to a preliminary distillation and. gave 11 pounds oftoluic acid fraction contaminated with aliphatic acids as well as theusual neutral oils. The product wasl melted and poured into 9 litersofpentanewith stirring. The first crystalline fraction removed amountedto 3.5: pounds and had the following analysis:

Neutral equivalent 136.5 Benzoi'c' acid percent 2.0 OrtlIo-rto'luic aciddo 5.0 Meta-tol'uic acid do 55-56 Para-toluic acid do- 37-38 Uponfurther cooling of the mixture, a second crystalline fraction wasrecovered and amounted to 6.0' pounds having the following analysis:

Neutral equivalent 136.5 Benzoic acid "percent" 3.0 Ortho-toluic acid rdo 13.0 Meta-toluic acid do 59-60 Para-ttoluic acid do 24-25 temperaturein the examples, it is not absolutely neces: sary that that bethe case,temperaturesbotheabove and below room temperature being operable. Itispreferred, however, that the extraction be carried. out. atartemperature not in excess. of about 50 C., in order that. excessiveamounts of the acid will not be dissolved in the-solvent, and, for thesame. reason and also in order to minimize solvent and solvent. handlingcosts, the amount of solvent used should: not. be too large. Theseconsiderations will be readily understood by those skilled in the 7 art.

I claim:

1. In a method for the purification of ttoluic, acids prepared by theliquid phase air oxidation of xylenes, the steps, comprising subjectingthe reaction product to a distillation operation to remove xylenes andobtain toluic acids admixed with neutral materials having a boilingrange close to that of the acids, crystallizing toluic acids, andwashingithe crystallized acids with a saturated liquid hydrocarbonhaving from 3 to 8 carbon atoms per molecule at a temperature not inexcess of about 50 C.

2. A method as in claim 1 in which the hydrocarbon is pentane.v

3. A method as in claim 1- in which the hydrocarbon is petroleum ether.

References Citedin the file of this patent.

UNITED STATES PATENTS" Great Britain May 24,

1. IN A METHOD FOR THE PURIFICATION OF TOLUIC ACIDS PREPARED BY THELIQUID PHASE AIR OXIDATION OF XYLENES, THE STEPS COMPRISING SUBJECTINGTHE REACTION PRODUCT TO A DISFILLATION OPERATION TO REMOVE XYLENES ANDOBTAIN TOLUIC ACIDS ADMIXED WITH NEUTRAL MATERIALS HAVING A BOILINGRANGE CLOSE TO THAT OF THE ACIDS, CRYSTALLIZING TOLUIC ACIDS, ANDWASHING THE CRYSTALLIZING ACIDS WITH A SATURATED LIQUID HYDROCARBONHAVING FROM 3 TO 8 CABRON ATOMS PER MOLECULE AT A TEMPERATURE NOT INEXCESS OF ABOUT 50*C.